The present application relates to an active pixel sensor with an embedded A to D converter. More specifically, the present application describes using a flash A to D converter that has a nonlinear aspect.
FIG. 1 shows standard input/output curves of a video monitor. Curve 100 is an ideal I/O characteristic which would be completely linear between input and output. However, it is well known that most monitors have a more realistic characteristic shown as curve 102. The lower end of the brightness scale has less gain. The upper end of the scale blooms and cuts off.
These characteristics lead to a known complementary correction being applied to the output of image devices. This correction usually has two components: a gamma (γ) correction at the lower end and knee correction at the upper end. Curve 104 shows these conventional corrections. The gamma correction increases the contrast at the lower end of the signal range to compensate for reduced gain at the lower end of the monitor responsivity characteristic. The knee correction extends the dynamic range of the monitor at the upper end.
These corrections can be done in many different ways. One correction uses nonlinear CMOS diodes which operate as nonlinear resistors. However, these processes are difficult to fabricate reliably in a CMOS process. Another way is by using a digital signal processor.
The correction must be applied at video rates, thus necessitating fast signal processing for digital output sensors.